Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus, a reading method for an image reading apparatus and an image forming system.
Description of the Related Art
There has been known an image reading apparatus configured to read image information in a main scanning direction of an original placed on an original table glass while a reading unit is conveyed in a sub-scanning direction. The image reading apparatus detects a size of the original based on the read image information, for example. Specifically, the image reading apparatus detects an original edge (original end) based on the image information to determine the original size based on a result of the detection.
However, in the detection method as described above, when dust, hair, or other dirt adheres to, for example, the original table glass or an original pressing member mounted to a back surface side (original table glass opposing surface side) of a platen (platen cover), false detection of the original edge may occur.
In order to address such a problem, an image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2001-036696 determines whether the read image information is from an original region candidate pixel or an original region outside candidate pixel, to thereby detect the original edge based on the number of repetitions of each candidate pixel. The image reading apparatus is based on a premise that the original pressing member, which is normally white, has a black color, or a light source is turned on under a state in which the platen is opened. This is because, with this premise, a luminance of a region outside the original region is decreased, and thus the determination between the original region candidate pixel and the original region outside candidate pixel is facilitated. In this method, however, it is required to change the configuration of the original pressing member, which is normally white, so that the region outside the original region is prevented from being blackened. Further, the light source is turned on under a state in which the platen is opened, and hence there still remains a problem in that the user feels dazzled.
An image analysis apparatus disclosed in Japanese Patent Application Laid-open No. 2009-171217 uses a known differential filter to convert image data of an original into edge image data, and detects continuity of an edge point in the edge image data, to thereby avoid the influence of the dirt. This image analysis apparatus still has a problem in that false detection may occur when a white streak caused when a shading white plate gets dirty appears in an image at a position on the outer side of the original edge in the main scanning direction. This false detection occurs because the white streak appears in the image as a straight line, and continuity is determined to be present.
The present invention has a primary object to provide an image reading apparatus capable of preventing a user from feeling dazzled, and also capable of detecting a size of an original with high accuracy.